Thermoplastic powders or slurries, known as dry or liquid plastisols, are conventionally cast on the inner surface of a thin metal shell or tool which is heated to the fusion temperature of the deposited material to form a gel coat which is heated sufficiently to form a thin, solid thermoplastic molded member (i.e. a skin). The thin metal mold shell remains heated until a thin layer of thermoplastic material is built up on the front or inner surface to form a thin plastic product, such as a product for use as an outer covering on automotive interior products such as door panels, instrument panels, or the like.
The apparatus and method for the production of molded skins are well known, such methods often being referred to as "rotational" or "slush" molding. The mold cavity defined by the mold tool is filled with a flowable plastic material and, due to heating of the mold shell, a thin layer of plastic material adheres to the surface of the mold tool and the remaining plastic material can then be shaken or poured out of the mold tool, as by rotating same. When the mold tool is further heated, the product clinging thereto gels out. The finished product can then be removed after'the mold tool cools. The operation of introducing the plastic material, when in the form of a powder or granule, is typically achieved by rotating the mold tool so that the mold cavity thereof opens downwardly, and then fixing onto the mold tool a box which contains the powdered plastic material. The mold tool and powder box are then rotated through about 180.degree. so that the plastic material drops into the mold tool. After the plastic skin has set against the heated tool surface, the assembly is rotated back to its original position and the powder box and excess powder removed, with further heating of the mold tool effecting proper melting and gelling of all of the plastic material adhered to the tool surface. The mold tool is then cooled and the molded skin product removed.
In a typical rotational molding apparatus of the type described above, the mold assembly includes a generally hollow housing or box on which the metal mold tool is mounted, which housing and tool cooperate to define a hollow interior chamber to which heated air or fluid is supplied to effect heating of the mold tool from the rear surface thereof. More specifically, heated air or liquid is continuously supplied into one end of the mold housing and is discharged from the opposite end. This method of heating the mold tool, however, has long presented a problem since the irregularity in the shape of the mold tool and the length thereof have often made it difficult to provide the desired degree of heat uniformity as applied to the mold tool throughout the rear surface thereof. The mold tool thus often has undesired hot or cold spots which effect the quality (i.e. thickness and/or finished surface properties) of the molded skin product.
In an attempt to provide better control over the heat applied to the back surface of the mold tool, various flow arrangements have been proposed for controlling the flow of air through the mold housing. However, in these arrangements, temperature and velocity drops occur from one end of the mold housing to the other as the air moves therethrough, and this can result in unsatisfactory and uneven heating of the mold shell. In addition, the air flow control devices make the overall process more costly from a manufacturing standpoint.
In the arrangements which require pumping of heated liquid, such as oil, into one end of the mold housing and out of the opposite end thereof, temperature and velocity drops also occur as the heated liquid moves through the mold housing, causing nonuniform heating of the tool and resulting in a less than satisfactory product.
Further, in both types of rotational molding arrangements discussed above, pressure is utilized to fill the mold housing with fluid and suction is often utilized to discharge the fluid from the mold housing, and this typically requires the use of rotating pressure joints where the supply and discharge conduits connect to the mold housing. These types of joints significantly add to the expense and complexity of the arrangement and are therefore disadvantageous. In addition, the pressurized fluid stresses the mold tool and may shorten the useful life thereof.
Still further, in the known arrangements employing both heated air or heated oil for heating the tool, it is typical to provide the interior chamber of the mold housing with a matrix of flow tubes which are oriented transversely toward and terminate adjacent the rear surface of the tool so as to direct the heated air or oil toward the rear surface. The tubes defining this matrix are typically sized so that the discharge ends are all disposed close to but spaced generally a uniform distance from the rear surface of the tool. The discharge ends of the tube matrix thus define a profile which generally corresponds with the rear surface of the tool. This tool matrix is not only heavy and structurally complex, but is also configured to cooperate solely with a single tool shape, and hence is not adaptable to other tool shapes.
Also, the known molding arrangements employing hot oil for heating the tool typically require a pressurized supply system for the hot oil which not only increases the overall structural complexity of the system, but inherently decreases the overall safety of the system since even accidental minute leakage of pressurized hot oil can create a potential hazard to personnel working in the vicinity of the apparatus.
The present invention thus relates to an improved molding apparatus and method for forming a thin skin product which significantly improve upon the known apparatus and methods as briefly described above.
It is an object of the present invention to provide an improved molding apparatus and method for forming a thin skin product, which improved method and apparatus is believed to significantly improve the overall performance characteristics of the apparatus and method in terms of efficiency and rate of productivity. This invention provides more efficient heating of the rear surface of the mold tool by utilizing a mold arrangement capable of providing heated liquid in a more uniform manner to the rear surface of the mold shell, which results in more efficient and uniform heating thereof. In addition, the molding apparatus according to the invention is structurally simple and utilizes a batch of liquid within a mold tank to heat the rear surface of the mold tool, as compared to the above conventional methods which continuously flow heated fluid past or over the mold shell to heat same. Further, heating devices are provided within the mold tank and serve to maintain the liquid within the mold tank at a uniform temperature.
It is a further object of the present invention to provide an improved apparatus and method, as aforesaid, which due to the improved heating of the tool is capable of providing a more uniform and higher quality molded product in terms of uniform but thin thickness of the material defining the product, and which is capable of producing such product in a repetitive fashion as the method and apparatus are used in a typical production cycle.
A still further object of the invention is to provide an improved apparatus and method, as aforesaid, which in a preferred embodiment utilizes hot oil for heating the tool but employs a system whereby the hot oil is confined and moved about within an arrangement which is substantially nonpressurized so as to significantly improve the operational safety of the arrangement. This lack of a pressurized oil system is also believed to create less tool distortion during the molding cycle and thus improve tool life.
In the apparatus and method according to the present invention, particularly in a preferred embodiment, a generally hollow mold housing has an opening which mounts therein a mold tool so that a rear surface of the mold tool cooperates with the housing to define an interior chamber. A batch of preheated oil is supplied to this chamber to substantially cover the rear surface of the mold tool. A plurality of heaters, such as electric heater rods, are disposed in the interior chamber to supply makeup heat to the oil to maintain a uniform temperature thereof and effect uniform heating of the mold tool. The mold housing is preferably angularly oscillated through several cycles to effect agitation of the oil both to maintain uniformity of temperature thereof, and to cause the heated oil to move along the back surface of the mold tool to effect more efficient heating thereof. After heating of the tool a powder box is attached to the tool so as to permit powder to be supplied to and deposited on the front surface, whereupon the powder gels and melts to form a desired thin-walled product. After the powder box is detached, the hot oil is removed from the interior chamber, preferably by rotating the mold box into a partially inverted condition to permit gravity discharge of hot oil into a collecting tank. The mold box is returned to its upright position and cooling oil is then supplied into the interior chamber, and the mold box is preferably again cycled through several angular oscillations to agitate the oil in contact with the back surface of the mold tool to facilitate cooling of the tool and of the molded product attached thereto. The cooling oil is then removed, such as by partially inverting the mold to permit gravity discharge of cooling oil into an appropriate reservoir or holding tank. The product can then be removed from the mold tool, and the mold arrangement repositioned to initiate a new cycle by supplying heated oil to the interior chamber. The heated oil is preferably supplied to the interior chamber of the mold housing by a system which supplies the oil under a generally non-pressurized state to permit safe handling thereof, and the hot oil as supplied to the interior chamber is supplied substantially uniformly over the longitudinal length of the chamber to provide improved temperature uniformity throughout the entirety of the chamber.
Other advantageous structural features of the inventive apparatus, and advantageous operational features of the inventive method, are described in detailed hereinafter.
The advantageous apparatus and method of the present invention, and the objects and purposes thereof, will be apparent to persons familiar with molding of this general type upon reading the following description and inspecting the accompanying drawings.